High aspect ratio microstructures and methods for manufacturing microstructures

ABSTRACT

A method is disclosed for the manufacture of microstructures and devices. The method is relatively easy to implement, and has the capability to produce features having a resolution of ten microns or smaller with a high aspect ratio (60, 75, 100, 200, or even higher). A master mask, appropriately designed and fabricated, is used in an initial exposure step with visible light, ultraviolet light, x-rays, an electron beam, or an ion beam to make a &#34;transfer mask&#34; directly on the surface of the sample. It is not necessary to produce an expensive x-ray master mask, even if x-ray exposure of the sample is desired. There is no necessity for gap control during exposure of the resist through the transfer mask. The resulting structures may, if desired, have a higher aspect ratio than microstructures that have previously been produced through other methods. The &#34;transfer mask&#34; is not a unit separate from the sample, but is formed directly on the surface of each sample. A conventional-type master mask is used to form the &#34;transfer mask&#34; with visible light, ultraviolet light, x-rays, an electron beam, or an ion beam. The total cost is determined primarily by the cost of the master mask. Because the master mask can be a conventional-type optical mask, the high cost of producing a conventional x-ray mask can be avoided. The &#34;master mask&#34; is used to form a &#34;transfer mask&#34; on each sample individually. The patterned transfer mask comprises a thin layer of an absorber of the radiation to be used in the final exposure. For example, if the final exposure is to be performed with soft x-rays, the transfer mask may be formed from an x-ray absorber such as a patterned layer of gold. The transfer mask is then used in one or more separate exposures of the underlying resist. An analogous method may be used for radiation-assisted chemistry, such as etching or deposition, on the surface of a sample.

TECHNICAL FIELD

This invention pertains to microscopic machines, structures, anddevices, particularly to microscopic machines, structures, and deviceshaving a high aspect ratio, and to methods of making such machines,structures, and devices.

BACKGROUND ART

Microscopic machines, structures, devices, and integrated circuits(hereafter collectively called "microstructures" for simplicity) havewide application. Integrated circuits are used in devices too numerousto be recited. Microstructures other than integrated circuits, whosedimensions are typically on the order of several hundred microns down toone micron, or even into the submicron range, also have a wide range ofapplications. They have been used in micromechanics, microoptics,integrated optics, sensors, actuators, and chemical engineering.Microstructures that have been built include such structures as gears,nozzles, chromatographic columns, acceleration sensors, microturbines,micromotors, and linear actuators.

Microstructures are usually manufactured through a lithography process.In lithography, one or more "masks" are initially prepared, each maskincorporating all or part of the pattern to be formed on a samplesurface. Transparent and opaque areas of the mask represent the desiredpattern. Radiation, such as visible light, ultraviolet light, x-rays, anelectron beam, or an ion beam, is transmitted through the mask onto aresist. After exposure, the resist (which may have either a positivetone or a negative tone) is developed to form the pattern on the samplesurface.

To support the opaque portions of the pattern in the mask, a substrateor carrier is used that is reasonably transparent to radiation at thewavelength used for the exposure. For lithography in visible orultraviolet wavelengths, glass has typically been used as the carrier.For x-ray masks, carriers have typically been expensive membranes a fewmicrons thick, usually made of a low-Z ("Z"=atomic number) material suchas silicon, beryllium, titanium, aluminum, silicon nitride, or graphite.

In making an integrated circuit, it is usually desirable to have the"depth" of a feature (i.e., the dimension in the direction normal to thesurface of the pattern) be relatively small. By contrast, in makingmicrostructures other than integrated circuits, it is often desirable tohave the "depth" of the feature be relatively large (i.e., deep-etchlithography), to impart a three-dimensional structure to themicrostructure, or a reasonable degree of strength to themicrostructure, or both.

The resolution of a microstructure is the dimension, in a directionparallel to the structure's surface, of the smallest reproduciblefeature, or the smallest reproducible gap between adjacent features. The"aspect ratio" of a microstructure is the ratio of the depth of afeature to the resolution. To the inventors' knowledge, an aspect ratioof about 50 is the highest aspect ratio that has previously actuallybeen achieved for any microstructure having a resolution of 10 micronsor smaller.

Three methods of imaging have previously been used in lithography. Inproximity imaging, the mask is positioned a small but finite distance(or gap) from the sample surface. Proximity imaging is predominantlyused in x-ray lithography.

In projection imaging, a projection lens is placed between the mask andthe sample to focus light onto the sample surface. (Alternatively, acondenser lens may be placed before the mask.) Projection imaging ispredominantly used in visible and ultraviolet optical lithography.

In contact (or zero-gap) printing, the mask is placed directly on (butnot adhered to) the sample surface. Contact printing is rarely used inlithography. In x-ray lithography, contact printing can result inserious contamination problems for the sample, and in degraded integrityfor the mask. In optical lithography, contact printing can result indeterioration of the image from uncontrolled multiple reflections andlight interferences between the mask and the sample surface.

Producing masks for lithographic applications can be expensive. X-raymasks are particularly expensive, typically costing about three to fivetimes as much as an optical mask. The cost of mask production has twoprincipal components: the cost of the substrate, and the cost of patternformation. For a master mask having a moderate-density pattern, thesetwo cost components are often roughly equal. But for a high-densitypattern, such as is typical of a mask for an integrated circuit, thecost to produce the pattern can be many times higher than the cost ofthe substrate. On the other hand, for masks used in x-raymicromachining, the cost of the substrate is typically higher. In thelatter case, the substrate provides the rigidity and integrity needed bythe mask, which is subjected to the heavy impact of high doses ofionizing radiation.

"LIGA--Movable Microstructures," Kernforschungszentrum Karslruhe (1993?)reported the manufacture of an acceleration sensor having a height of100 microns, a cantilever 10 microns wide, and slit width of 4 microns,i.e., an aspect ratio of 25. See the left-hand column of page 8 of thatpublication.

J. Mohr et al., "Herstellung von beweglichen Mikrostrukturen mit demLIGA-Verfahren," KfK Nachrichten, Jahrgang 23, 2-3/91, pp. 110-117(1991) reported the manufacture of a structure having a gap width of 3microns, and a depth of 150 microns, i.e., an aspect ratio of 50. SeeFIG. 6 of Mohr et al.

E. Spiller et al., "X-Ray Lithography," Research Report, I.B.M. T. J.Watson Research Center (1977) disclosed a technique for x-ray microscopyof biological objects in which a specimen is brought in close contactwith the resist surface, either mounted on a thin substrate, a grid, ordirectly on top of the resist layer. After development a reliefstructure is obtained in which the heights of a feature in the resistcorrespond to the x-ray absorption of the specimen. A scanning electronmicroscope was used to produce a magnified picture of the resist reliefimage. See pp. 48-49 and FIGS. 3.28 and 3.29 of Spiller et al.

DISCLOSURE OF THE INVENTION

A novel method has been discovered for the manufacture ofmicrostructures. The novel method is relatively easy to implement, andhas the capability to produce features having a resolution of tenmicrons or smaller with a high aspect ratio (60, 75, 100, 200, or evenhigher). A master mask, appropriately designed and fabricated, is usedin an initial exposure step with visible light, ultraviolet light,x-rays, an electron beam, or an ion beam to make a novel "transfer mask"directly on the surface of the sample. It is not necessary to produce anexpensive x-ray master mask, even if x-ray exposure of the sample isdesired. There is no necessity for gap control during exposure of theresist through the transfer mask. The resulting structures may, ifdesired, have a higher aspect ratio than microstructures that havepreviously been produced through other methods.

Briefly, the "transfer mask" used in the novel method is not a unitseparate from the sample, but is formed directly on the surface of eachsample. A conventional-type master mask is used to form the "transfermask" with visible light, ultraviolet light, x-rays, an electron beam,or an ion beam. The total cost is determined primarily by the cost ofthe master mask. Because the master mask can be a conventional-typeoptical mask, the high cost of producing a conventional x-ray mask canbe avoided.

The "master mask" is used to form a "transfer mask" on each sampleindividually. The patterned transfer mask comprises a thin layer of anabsorber of the radiation to be used in the final exposure. For example,if the final exposure is to be performed with soft x-rays, the transfermask may be formed from an x-ray absorber such as a patterned 3-micronlayer of gold. The transfer mask is then used in one or more separateexposures of the underlying resist.

Using soft x-rays with a wavelength around 1 nm, such a transfer maskhas been used to form high resolution features more than 200 micronsdeep, but having a gap width as fine as 2 micron, i.e., an aspect ratioof 100. Seven stages of exposure and development were used to createthese features, each stage adding a depth of about 30 microns. Ifdesired, additional stages of exposure and development would allow theformation of features having a depth up to 500 microns, or even deeper.(By contrast, a depth of only about 2 microns is possible with opticallithography.) If desired, different exposure steps could be evenperformed with different wavelengths of incident radiation.

The resolution may be made substantially finer than 2 microns ifdesired--1 micron, 0.5 micron, 0.2 micron, 0.1 micron, or even smaller,depending primarily on the resolution of the transfer mask. A resolutionof 2 microns has been demonstrated to date, simply because that was theresolution of a readily available master mask. A master mask with finerresolution (which may readily be made through means known in the art)would result in a transfer mask, and therefore a microstructure, havingfiner resolution.

With a 0.1 micron resolution (in a transfer mask generated, for example,with an e-beam), the feature depth could be as great as 30 to 50 micronsbefore being limited by diffraction, i.e., an aspect ratio of 300 to500.

Because the transfer mask is attached to the resist, there is no need toalign the transfer mask and the surface, nor is there any need for gapcontrol between the transfer mask and the surface. Because there is zerogap between the transfer mask and the sample, feature resolution closeto the theoretical limit for a given wavelength of radiation may beachieved, minimizing the diffraction and blurring that a gap inevitablyintroduces. Multiple stepwise exposure and development, as previouslydiscussed, can also help reduce the effects of diffraction. The resistunderlying the absorber (see, e.g., FIG. 1(i)), helps absorb diffractedradiation, reducing its impact.

The transfer mask approach optionally permits the use of new lithographytechniques, such as in situ development (simultaneous exposure anddevelopment), or radiation-assisted chemistry (either etching ordeposition). In a conventional mask, the presence of a mask substratewould interfere with in situ development. Similarly, the presence of amask substrate is incompatible with radiation-assisted chemistry,because of the physical obstacle that the substrate imposes.

In situ development is that which occurs simultaneously as exposureoccurs, without the necessity of a separate development step. An exampleof a prototype of an in situ development process is the "sublimation" ofpolymethylmethacrylate ("PMMA") from the surface when PMMA is exposed tohigh doses of x-rays or ultraviolet light in vacuum, or where PMMA isexposed to x-rays or ultraviolet light in the presence of oxygen.

In radiation-assisted chemistry, a material other than a resist isetched or deposited in a manner somewhat similar to reactive ion etching(RIE). In reactive ion etching, a radio-frequency is used to excite agas to a plasma state; and the resulting ions in the plasma then etch asurface. In radiation-assisted etching, a gas is ionized in the vicinityof the site of radiation exposure, leading to etching directly on thesurface, without the use of a resist. Examples are the etching of asilicon surface, or of a gallium arsenide surface, with a gas comprisinga chlorocarbon, a chlorofluorocarbon, oxygen, chlorine, or fluorine.

Forming a transfer mask directly on the sample opens new possibilitiesthat are not available with conventional masks, for example, exposure ofsamples with curved surfaces, dynamic deformation of the sample surfaceduring exposure, and rotation of the sample during exposure. Oneapplication of such techniques is the creation of three-dimensionalstructures whose cross-section can vary as a function of depth.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a) through 1(j) illustrate a process of making a microstructurein accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Fabrication of the master mask may be performed through known methodsfor preparing an optical, ultraviolet, x-ray, e-beam, or ion-beam mask.The master mask is then used to prepare the transfer mask using standardlithography techniques. Depending on the resolution required, thetransfer mask may be fabricated from the master mask with optical,ultraviolet, x-ray, e-beam, or ion-beam lithography techniques.

The transfer mask comprises an absorber pattern formed directly on thesurface of the resist layer of the sample to be patterned. The transfermask is used in subsequent steps to form the pattern in the sample.Either an additive or a subtractive method may be used.

Formation of the Transfer Mask--Additive Method

As illustrated in FIGS. 1(a) and 1(b), a three-layer starting materiallying atop a substrate 1 may be used. Layer 2, immediately adjacentsubstrate 1, is the sample resist, which can be any one of many knownresists, such as PMMA or an AZ-type resist (available from HoechstCelanese or from Shipley Co.). Layer 3, immediately adjacent layer 2, isthe transfer mask plating base. Plating base 3 preferably comprises anelectrical conductor, for example a 300-500 Å layer of gold. Layer 4,immediately adjacent to layer 3, comprises a resist of thicknesssufficient to form the desired transfer mask absorber (for example, a 3to 10 micron thickness of a commonly used AZ-type resist.)

As shown in FIG. 1(c), transfer mask resist 4 is exposed through mask 5with radiation (designated "hν"), which may be visible, ultraviolet, orx-ray radiation, or an e-beam or ion beam through master mask 5. Forexample, this step may comprise proximity printing with an opticalmaster mask. Alternatively, the transfer mask resist may be exposeddirectly with an e-beam writer (not illustrated). Transfer mask resist 4is then developed (FIG. 1(d)), and transfer mask absorber 6 is thenformed (FIG. 1(e)), for example by electroplating a 3 to 8 micron layerof gold onto plating base 3, in the areas of transfer mask resist 4 thatare removed during development. (The transfer mask thickness is thatwhich is appropriate for the amount of radiation to be used to exposethe sample resist to a given depth during one exposure step. Dependingon the intended application, the transfer mask absorber might, forexample, have a thickness between 0.5 micron and 50 microns.) Althoughgold is the preferred material for absorber layer 6 in this additivemethod, other absorbers such as nickel or copper could also be used.

Then the remaining transfer mask resist is removed (FIG. 1(f)), forexample by blanket exposure and development of the remaining resist.Transfer mask plating base 3 is removed from areas not corresponding tothe transfer mask pattern, preferably by dry etching in an anisotropicmode that will etch preferentially in a direction normal to the surface,for example by argon ion milling. Alternatively, wet etching with anappropriate solvent may be used; for example, a solution of I₂ inaqueous potassium iodide will remove gold at a controllable rate (e.g.,0.1 to 1.0 micron per minute, depending on concentration andtemperature).

Formation of the Transfer Mask--Subtractive Method (Not Illustrated)

In some applications, it may be desirable to form a transfer mask havinga reversed, or negative, tone. The transfer mask absorber layer, forexample a few micron thickness of tungsten, is applied to the sample tobe patterned, for example by chemical vapor deposition. Then a resist isapplied of sufficient thickness (e.g., a 1-2 micron AZ type resist) toform a transfer mask absorber having appropriate thickness. The transfermask resist is exposed through the master mask, followed by developmentof the transfer mask resist. The transfer mask is then formed by dryetching, wet etching, or reactive ion etching through the developedportions of the transfer mask resist. Although tungsten is the preferredmaterial for the absorber layer in the subtractive method, otherabsorbers such as gold, nickel, or copper could also be used.

Patterning with the Transfer Mask

The layer underlying the transfer mask may be a resist, such as PMMA,AZ, or alternatively it could be another material that can be processedwith radiation-assisted chemistry, such as SiO₂.

Sample resist 2 (with adhering transfer mask 6) is exposed to radiationof a chosen wavelength--visible, ultraviolet, or x-ray--or to anelectron beam or ion beam if desired (FIG. 1(g)). For many applications,collimated x-rays (e.g., x-rays from a synchrotron) will be thepreferred radiation source.

There are many possible ways to expose the sample with its adheredtransfer mask. Exposure may be stationary. The exposure may be performedduring scanning of either the radiation source or the sample. The angleof exposure may be normal to the surface, or any other incident angledesired. The angle of exposure may be constant, or variable, includingrotation or wobbling. The surface of the sample may be flat or curved.If a curved surface is desired, the curvature is preferably appliedafter the transfer mask has been formed on the sample; in such a case,it may be appropriate to distort the pattern on the master mask in sucha way that the transfer mask can be printed flat, but will result in thedesired pattern on the surface after the surface has been curved intothe desired shape. The surface shape may remain constant duringexposure, or it may even be changed dynamically during exposure.

Development of the resist (either wet or dry development) may followexposure as an independent step. Alternatively, in situ development(simultaneous exposure and development) can occur in an appropriateatmosphere (vacuum or gas mixture).

In an alternative embodiment, radiation-assisted chemical processing canbe used in chemical etching of the sample, or chemical deposition ontothe sample.

The sample may be exposed in a single step, or in a series of multiplesteps with intervening development steps (FIGS. 1(g)-(j)). Usingmultiple steps can permit the creation of features having a greaterdepth than can be achieved in a single, one-step exposure. Becausetransfer mask 6 adheres to sample resist 2, the transfer mask isself-aligning, meaning that there is no need to maintain alignment ofthe mask and sample between exposure steps.

Either positive or negative tone patterns on the transfer mask may beused in one-step processing. However, positive tones are preferred formultiple-step processing. With a negative tone, it will generally bedesirable to remove the transfer mask by dry or wet etching beforedevelopment.

Graded tone patterns may also be used, and are suitable fordifferentially shaping the resist in a direction normal to itssurface--for example, a hemispherical feature or other three-dimensionalfeature could be made with graded toning.

After development, the transfer mask may be removed (if desired) by dryor wet etching. Dry etching removal, for example, may be performed byargon ion milling. Wet etching removal of gold, for example, may beperformed with a solvent such as a solution of I₂ in aqueous KI.

FIGS. 2, 3, and 4 (which are not reproduced here, but which may be seenin the published international application, WO 9/07954, internationalpublication date Mar. 14, 1996) are electron micrographs illustrating aresist pattern of a microstructure that has been manufactured inaccordance with the present invention. (Scale bars are included in eachphotograph.) A self-supporting 1.5 mm thick PMMA resist sheet waspatterned to a depth of slightly over 200 microns, with a minimum gap of2 microns. FIG. 2 is an overview of the entire test pattern, which waschosen to represent a typical micromechanical structure. FIG. 3 is anenlarged view of the far left "spoke" of this pattern. FIG. 4 is afurther enlargement of the top of the spoke illustrated in FIG. 3.

The transfer mask plating base was prepared by evaporating onto theresist 70 Å of chromium (as an adhesion promoter), followed by 500 Å ofgold, using an electron beam evaporator. A transfer mask resistcomprising a 3 micron-thick layer of standard novolak-based AZ-typeresist S1400-37 (Shipley Co.) was formed on the plating base byspinning.

The resist was then exposed with ultraviolet light through an opticalmask in proximity mode with a gap of 12 microns. After exposure, theresist was developed with a standard developer, Microposit 454 (ShipleyCo.). Exposure and development were performed following themanufacturer's recommended procedures.

After development, the transfer mask was formed by electroplating a 3micron layer of gold onto the plating base, using a standard goldelectroplating solution (Enthone-OMI Co.). The transfer mask resist wasthen removed by blanket exposure and subsequent development. The exposedplating base was then etched for 20-30 seconds with a solution of KI (5%by weight) and I₂ (1.25% by weight) in water. The chromium layer wasremoved by etching in a standard chromium etch (KTI Co.).

X-ray exposures were made with the x-ray lithography beam line at the J.Bennett Johnston Sr. Center for Advanced Microstructures and Devices,Louisiana State University, Baton Rouge, La. The beamline was equippedwith two grazing-incidence (1.5°) flat mirrors, and provided collimatedsoft x-ray radiation with a wavelength of 7-11 Å, optimized formicrocircuit fabrication. The PMMA resist was patterned stepwise throughthe transfer mask. Each step comprised an both exposure and subsequentdevelopment. The exposure dose was 8 to 12 J/cm². A five minutedevelopment removed 30 microns of exposed PMMA. Development wasperformed in a standard developer, namely that of W. Ehrfield et al.,"Progress in Deep-Etch Synchrotron Radiation Lithography," J. Vac. Sci.Tech., vol. B 6, No. 1, pp. 178-182 (1988). The exposure/developmentcycle was repeated seven times. The seven-step process resulted in apattern a little over 200 microns deep. The finest features (gaps) were2 microns wide, corresponding to an aspect ratio of 100. See FIGS. 2, 3,and 4.

Miscellaneous

As used in the claims, the term "radiation" is intended to encompass atype of radiation to which a resist (or other material in a sample) issensitive. For example, unless the context indicates otherwise,"radiation" could comprise visible light, ultraviolet light, softx-rays, hard x-rays, an electron beam, or an ion beam.

The entire disclosures of all references cited in this specification arehereby incorporated by reference in their entirety. In the event of anotherwise irresolvable conflict, the present specification shall controlover a document incorporated by reference.

What is claimed is:
 1. A method for fabricating a selected pattern atleast 6 microns thick in a resist at least 6 microns thick; wherein thepattern comprises at least one feature having a resolution of 10 micronsor smaller and having an aspect ratio of 60 or greater; and wherein saidmethod comprises the steps of:(a) forming a transfer masklithographically, wherein the transfer mask adheres to the surface ofthe resist, wherein the transfer mask comprises portions that correspondto the pattern and that are opaque to radiation to which the resist issensitive, wherein the transfer mask comprises other portions that aretransparent to radiation to which the resist is sensitive, wherein theopaque portions corresponding to the pattern have either a positive toneor a negative tone relative to the pattern, wherein the resist has apositive tone if the opaque portions of the pattern have a positive tonerelative to the pattern, wherein the resist has a negative tone if theopaque portions of the pattern have a negative tone relative to thepattern; (b) exposing the resist and the transfer mask to radiation towhich the resist is sensitive, whereby portions of the resist underlyingthe opaque portions of the transfer mask are not exposed to theradiation, and whereby portions of the resist underlying the transparentportions of the transfer mask are exposed to the radiation; and (c)developing the resist to selectively remove the radiation-exposedportions of the resist if the resist has a positive tone; or toselectively remove the unexposed portions of the resist if the resisthas a negative tone; whereby the pattern is formed in the resist.
 2. Amethod as recited in claim 1, wherein the resist has a positive tone,wherein the opaque portions of the pattern have a positive tone relativeto the pattern, and wherein the opaque portions of the transfer maskcomprise gold.
 3. A method as recited in claim 1, wherein the resist hasa negative tone, wherein the opaque portions of the pattern have anegative tone relative to the pattern, and wherein the opaque portionsof the transfer mask comprise tungsten.
 4. A method as recited in claim1, wherein said exposing step and said developing step are each repeateda plurality of times, and wherein the transfer mask is in the sameposition relative to the resist during each of said exposing steps.
 5. Amethod as recited in claim 1, wherein during said exposure step theposition of the resist is scanned relative to the position of theradiation, or wherein during said exposure step the position of theradiation is scanned relative to the position of the resist.
 6. A methodas recited in claim 1, wherein during said exposure step the position ofthe resist is rotated relative to the position of the radiation, orwherein during said exposure step the position of the radiation isrotated relative to the position of the resist.
 7. A method as recitedin claim 1, wherein the transfer mask additionally comprises portionsthat are semi-opaque to radiation to which the resist is sensitive,whereby in said exposing step portions of the resist underlying thesemi-opaque portions of the transfer mask are partially exposed toradiation; and whereby in said developing step the partially-exposedportions of the resist are removed to a lesser depth than the depth towhich the fully-exposed portions of the resist are removed.
 8. A methodas recited in claim 1, wherein the radiation comprises x-rays.
 9. Amethod as recited in claim 1, wherein the radiation comprises visiblelight.
 10. A method as recited in claim 1, wherein the radiationcomprises ultraviolet light.
 11. A method as recited in claim 1, whereinthe radiation comprises an electron beam.
 12. A method as recited inclaim 1, wherein the radiation comprises an ion beam.
 13. A method asrecited in claim 1, additionally comprising the step of removing thetransfer mask after the pattern is formed in the resist.
 14. A method asrecited in claim 1, wherein the angle of incidence of the radiation onthe surface of the transfer mask is 90°.
 15. A method as recited inclaim 1, wherein the angle of incidence of the radiation on the surfaceof the transfer mask is less than 90°.
 16. A method as recited in claim1, wherein the angle of incidence of the radiation on the surface of thetransfer mask changes with time during said exposing step.
 17. A methodas recited in claim 1, wherein the surface of the transfer mask and thesurface of the resist are not flat.
 18. A method as recited in claim 1,wherein the shape of the transfer mask and the shape of the resistchange with time during said exposing step.
 19. A method as recited inclaim 1, wherein the pattern fabricated in the resist comprises at leastone feature having a resolution of 10 microns or smaller and having anaspect ratio of 60 or greater.
 20. A method as recited in claim 1,wherein said exposing step and said developing step are each repeated aplurality of times; wherein the transfer mask is in the same positionrelative to the resist during each of said exposing steps; and whereinthe angle of incidence of the radiation on the surface of the transfermask changes between said exposing steps.
 21. A method as recited inclaim 1, wherein said exposing step and said developing step are eachrepeated a plurality of times, and wherein the shape of the transfermask and the shape of the resist change between said exposing steps.